The Basics of Creating Terrain Models
By Brian Morse, August 10, 2004
This article covers the sequence of commands used to create a terrain model using Land Desktop. It also provides some quick ways to visualize your terrain model in 2D and 3D.
The terms surface and terrain model are use interchangeably. A terrain model is a collection of 3D points and the relationship between adjacent points within the model. There are many types of data you can use to build a terrain model. Points, 3D Polylines, and Contours are the most common sources of data. For your first time through the process of building a surface using Land Desktop it is best to illustrate the process with a very limited example, sampling Land Desktop Point Objects from a drawing.
1. From the Terrain pull-down choose the Terrain Model Explorer command.
2. Right Clicking is the only way to launch most commands in Terrain Model Explorer (TME). Some commands are found nowhere else except within TME. Right Click on the Terrain folder in TME and choose Create Surface.
3. Rename the surface by right clicking on the surface name (the default name is Surface1).
Use a name which is appropriate such as EG for existing ground or FG for finished ground.
Bear in mind that the surface name can be used as a Layer (name) Prefix. In this example we will use EG for the surface name.
4. When you left click on the Terrain Folder within TME the right side displays all the surface names.
Look at the Status column. This is a good way to keep informed of where you are in the process used to build surfaces. In this example the Status column should now display "No Data" for your surface.
5. Left click the plus signs on the left side of TME to expand the list of available commands to add TIN Data for your surface. Under TIN Data you will see the categories Point Files, Point Groups, DEM Data, etc.
6. For the remainder of the example shown in class we focused on building a surface from point data in an existing drawing. This is one of many ways to add data to a surface.
Right click on Point Files under TIN Data in TME and choose Add Points from AutoCAD Objects > Points. Point Objects fall under the category of AutoCAD Points in this context.
Select objects by [Entity/Layer] <Layer>:
7. Select Point Objects in the drawing.
Let’s pause to examine what is going on with the project data.
In Step 2 a folder is created under the DTM folder in the project data. When you Create a surface Land Desktop merely creates a folder. In Step 6 the sampled data is written to this folder.
Example of a path to a surface named EG within a project called 0027600902
8. Build the EG Surface by right clicking on the surface name in TME (Terrain Model Explorer… in case you forgot what TME means).
9. The Surface data options in the Build <surface> dialog box are used to select the type of data you wish to process when building a surface. If there is no sampled data for a specific data type it does not matter whether that specific option is checked or not. Click OK to build the surface. A message box displays when the process is complete.
The process for creating all surfaces in Land Desktop follows this sequence illustrated above. Now you may develop a better understanding of your data when you use Land Desktop’s features for displaying terrain models. It is time to visualize what you have created.
First we’ll try Quick View. This just paints vectors on the screen. A pan, zoom, or redraw will clear the TIN display from the screen. Quick View is fast and requires the least amount of work and does not add new entities to the drawing.
Looking at terrain models in 3D helps you to see if there are problems in your surface. Terrain models are only as accurate as the data you put into them. Knowing what part of your terrain model is accurate and what is not will help you decide whether it is necessary to fix the problems or not.
We used the Elevation Banding command in class to produce color coded 3D faces to help visualize the surface.
The Surface Elevation Shading Settings dialog is where you can enable the use of a Layer Prefix. The surface name will be used as the first characters in the name of the layer(s) created by Elevation Banding and other commands under the Terrain pull-down menu.
Rules of Terrain Modeling in Land Desktop.
There is only one surface current at a time.
Nearly all the commands under the Terrain pull-down menu act upon the current surface.
Terrain Model Explorer is the primary interface for creating surfaces.
Choose the Auto-Range button. A small dialog appears showing the upper and lower elevations for the current surface.
Elevation Banding is similar to contours except the band has an upper and lower elevation instead of a single elevation for a contour. The Surface Range Definitions dialog box is used to change elevation values for each range or color choices. The layer names are best left in their default state. This dialog box shows the suffix portion of the “Range Layers” created by this command.
Click OK to dismiss the Surface Range Definitions dialog box and return to the Elevation Shading Settings dialog. Click OK then focus your attention on the command line. Answer YES to Erase the objects on the “Range Layers”.
A Range Statistics dialog appears next. Click OK. Colored triangles should appear in drawing. These are 3D faces assigned a specific color and layer.
Now comes the fun part, using the 3D orbit command to dynamically rotate your view and apply shading to give your terrain model the appearance of being solid. The 3D Orbit command is found under the view pull-down menu. Typing 3DO on the command line will also start this command. Use the 3D Orbit command like you would a joystick. Start with your cursor positioned in the center of the green circle on your screen. Press and hold the left mouse button while slowly dragging your cursor straight up toward the top of your screen.
Navigating in 3D takes some practice. Hitting ESC will cancel the command and leave you with a 3D view on your screen. To get back to plan view type PLAN on the command line. Type “W” to get back to the plan view in the World coordinate system. Go back to the 3D Orbit command and right click while in this command. Choose Shading Modes > Gourard Shading. Whoa, pretty cool, eh? You can also control shading settings from the View pull-down menu. Choose Wireframe to turn off the shading.
With some appropriate choices in elevation ranges and colors you can make your models more intuitive to understand and easier to interpret.
The process illustrated here will get you started creating and using terrain models. Terrain Models open many doors to other features found in Land Desktop and Civil Design. With a Terrain Model you can use commands to create Profiles, Cross Sections, calculate Volumes, and perform design functions using Road Templates and Grading Objects.
About the Author
Brian Morse is a certified Autodesk instructor specializing in Land Desktop. He does training, consulting, contract drafting and design for Land Surveyors, Civil Engineers, Planners and Architects. Information about his services is available at www.BrianMorse.com.
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